When the request has completed loading it sets
readyState to 4 (Loaded). For a request of type HEAD, it sets the
readyState value to 4 immediately after setting it to 3.
The send() method takes an optional parameter that may contain data
of varying types. Typically, this method is used to send data to the server
using the POST method. The send() method may be explicitly invoked
with null, which is the same as invoking it with no argument. For most
other data types, set the Content-Type header using the
setRequestHeader() method (explained below) before invoking the
send() method. If the data parameter in the send(data) method is of
type DOMString, encode the data as UTF-8. If data is of type
Document, serialize the data using the encoding specified by
data.xmlEncoding, if supported or UTF-8 otherwise.
1.4.4 The setRequestHeader() Method
The setRequestHeader(DOMString header, DOMString
value) method sets request headers. This method may be called after
calling the open() method-when the readyState value is 1-otherwise
you'll get an exception.
1.4.5 The getResponseHeader() Method
The getResponseHeader(DOMString header, value)
method method is used to retrieve response header values. Call
getResponseHeader()only when the readyState value is 3 or 4
(in other words, after the response headers are available); otherwise, the
method returns an empty string.
1.4.6 The getAllResponseHeaders() Method
The getAllResponseHeaders()method returns all the response
headers as a single string with each header on a separate line.
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